(1) Field of the Invention
This invention relates to a method for continuously producing laminated sheets or films from a substrate sheet material and ultra-high molecular weight polyethylene (hereinafter sometimes simply referred to as "UHMWPE").
More particularly, the invention relates to a method for continuously producing laminated sheets or films from ultra-high molecular weight polyethylene which has a high melt viscosity and can be processed difficultly, and a substrate sheet material such as other various kinds of thermoplastic resins, rubbers, metals and cloths.
(2) Description of the Prior Art
Ultra-high molecular weight polyethylene has excellent impact strength and abrasion resistance. However, the consumption of this resin is very small because its processing is difficult.
Articles, sheets, and plates of ultra-high molecular weight polyethylene are formed by, for example, sintering, ram extrusion using a plunger pump, forging, and compression molding. However, it is very difficult to produce thin sheets or films (hereinafter referred to simply as "sheets") of less than 1 mm in thickness from ultra-high molecular weight polyethylene. According to the present industrial practice, thin sheets of this kind are produced by applying a secondary working such as skiving to a cylinder obtained by one of the above-mentioned methods. Therefore, with this method, the processing cost is high and continuous production of thin sheets on a large scale is quite difficult.
The difficulty in processing ultra-high molecular weight polyethylene arises from the fact that it has quite a high melt viscosity and has no good melt flow properties, and therefore, the conventionally employed tubular film method and T-die method in which the pressure drop is large, are not suitable.
Meanwhile, as a hitherto well known method for forming sheets, there is a calendering method. Because sheets of accurate and uniform thickness can be produced at a high rate, polyvinyl chloride sheets and rubber sheets are produced by this method. In the case of polyolefin resins, their melt strengths are low and their melt viscosities are largely dependent on temperature, so that the optimum range of their forming temperatures is narrow, which makes the forming operation difficult. Accordingly, this method is seldom employed for polyolefin resins.
Furthermore, as described above, the ultra-high molecular weight polyethylene is not only difficultly processable but also not susceptible to adhesives because it has no functional group. Accordingly, the preparation of laminated sheets with other substrate materials is considered to be very difficult.
As the methods for laminating the ultra-high molecular weight polyethylene with various substrate materials, there are proposed some methods in Japanese Laid-Open Patent Publication Nos. 57-207669, 58-20273, and 58-155918. However, it is difficult to produce continuously laminated sheets of uniform quality by any of the above methods.